We hope to continue a number of complementary studies of interactions between single neurons in both mammals and invertebrates. In each case we will record simultaneously the activities of several neurons. With intracellular recordings and PSP shape sorting we shall be examining synaptically related assemblies of neurons: the detailed anatomy of such neurons will be studied by fluorescent dye injection techniques. With extracellular recordings either from several electrodes or from a special electrode and spike-shape sorting, we shall be examining spatial assemblies of neurons. By using computer and mathematical techniques to examine spike trains and membrane potentials we can determine the functional organization of each observed neuronal group. By making appropriate changes in the stimulus or behavioral conditions of the experiments, we will be able to determine whether the organization or boundaries of a neuronal assembly vary with time or task. These approaches will be applied to neurons of: a) primary sensory cortex during passage of sensory information; b) thalamic structures during convergence of sensory information from two modalities; c) association cortex during discrimination behavior for which that particular cortex is essential; d) crayfish ganglia during behavioral conditioning.